1. Field of the Invention
The invention relates to a powertrain for a hybrid electric vehicle with an electro-mechanical reverse drive power flow path.
2. Background Art
A known hybrid electric vehicle powertrain with a so-called divided power characteristic, sometimes referred to as a power split “HEV”, comprises gearing, including a planetary gear unit, that establishes power flow paths from an internal combustion engine and an electric traction motor to vehicle traction wheels. An example of powertrains of this type are disclosed in U.S. Pat. No. 6,991,053, which is assigned to the assignee of the present invention, and U.S. Pat. RE38017.
The powertrains disclosed in these references include a planetary gear unit and an internal combustion engine connected directly to the carrier of the planetary gear unit. The sun gear of the planetary gear unit is drivably connected to a generator. A ring gear of the planetary gear unit acts as a power output member that delivers driving power to traction wheels through torque transfer gearing. A motor also delivers power to vehicle traction wheels through the transfer gearing. The generator and the motor, together with a high voltage battery, are electrically coupled.
The generator can be commanded to provide power assistance to the powertrain during launch of the vehicle. Electric power then is supplied to the battery, which in turn powers the motor.
During reverse drive, the motor may be operated in a reverse direction to provide a reverse torque to the vehicle traction wheels. During reverse drive, the generator is incapable of operating as a motor to provide power assistance since there is no reaction torque available to transmit generator torque to the ring gear, except for a small torque that may rotate the engine crankshaft.
Previous attempts have been made to enhance reverse drive performance by providing additional gearing in the torque flow path for the motor. An example of a powertrain of this type may be seen by referring to U.S. Pat. No. 7,128,677, which is assigned to the assignee of the present invention. A powertrain of that type would become much longer because of the space needed for the additional gearing that would be needed. It would not readily be capable of being packaged in a contemporary automotive vehicle environment. It is an objective of the present invention, therefore, to overcome shortcomings of known hybrid electric vehicle powertrains by providing an enhanced reverse driving torque function that will not require an increase in the length of the powertrain and that would be capable of being packaged in a conventional automotive vehicle environment while providing a mechanical reverse power flow path.